Access to scanners for magnetic resonance imaging (MRI) is typically limited by cost and by infrastructure requirements. Here, we report the design and testing of a portable prototype scanner for brain MRI that uses a compact and lightweight permanent rare-earth magnet with a built-in readout field gradient. The 122-kg low-field (80 mT) magnet uses has a Halbach-cylinder design that results in minimal stray field and requires neither cryogenics nor external power. The built-in magnetic-field gradient reduces the reliance on high-power gradient drivers, lowering the overall requirements for power and cooling, and reducing acoustic noise. Imperfections in the encoding fields are mitigated with a generalized iterative image-reconstruction technique that leverages prior characterization of the field patterns. In healthy adult volunteers, the scanner can generate T 1 -weighted, T 2 -weighted and proton-density-weighted brain images with a spatial resolution of 2.2 × 1.3 × 6.8 mm 3 . Future versions of the scanner could improve the accessibility of brain MRI at the point of care, particularly for critically ill patients.

Purpose We propose a constrained optimization approach for designing parallel transmit (pTx) pulses satisfying all regulatory and hardware limits. We study the trade-offs between excitation accuracy, local and global SAR, and maximum and average power for small flip-angle pTx (8 channels) spokes pulses in the torso at 3 T and in the head at 7 T. Methods We compare the trade-offs between the above-mentioned quantities using the L-curve method. We use a primal-dual algorithm and a compressed set of local SAR matrices to design RF pulses satisfying all regulatory (including local SAR) and hardware constraints. Results Local SAR can be substantially reduced (factor of 2 or more) by explicitly constraining it in the pulse design process compared to constraining global SAR or pulse power alone. This often comes at the price of increased pulse power. Conclusion Simultaneous control of power and SAR is needed for the design of pTx pulses that are safe and can be played on the scanner. Constraining a single quantity can create large increase in the others, which can then rise above safety or hardware limits. Simultaneous constraint of local SAR and power is fast enough to be applicable in a clinical setting.